Internet Service Provider (ISP) routinely peers with other ISPs to exchange traffic at specific locations to take advantage of communication patterns between peering ISPs. For instance, an ISP peers with another ISP in order to have direct communication and to exchange necessary traffic quickly. Additional advantages of peering include avoiding routing customer traffic through additional hops as well as reducing costs by agreeing to a symmetrical amount of data exchange, etc. Peering is voluntary interconnection of administratively separate Internet networks for the purpose of exchanging traffic between the customers of each network.
Large ISPs routinely filter incoming traffic that is viewed as unwanted. This filtering of incoming traffic is accomplished at considerable cost despite there is a significant upside ensuring that internal web sites stay up or data centers are not brought down, etc. Examples of unwanted traffic include spam emails, hacker attacks, probes, scans, etc. Such filtering is performed at firewalls using rules, access control lists, low-level traffic filters, and application-level software. ISPs also periodically examine outgoing traffic. In some cases, such examination is performed to prevent access to certain remote destinations. Examples could be blocked websites, known spyware or adware sites, or blacklisted destinations or Internet Protocol (IP) address prefixes. However, typically there is no effort to block outgoing traffic by ISPs.